Was shooting with my week-old 5DM3+50mm f/1.4 today and noticed alot of OOF shots in low-light, even after having AF tuned my lens to -1 with lens align the day I got the body. Put the camera back on my lens align station and found that its focus is still perfect at -1 in EV 8 lighting (f/1.4 1/160 ISO 100) but it front-focuses in EV 4-5 lighting (f/1.4 1/13 ISO 100). I then tried it with my 50L and saw the same thing. Has anyone seen this?

Here are my 50 f/1.4 images. In these images the EV 8 lighting is 5000k LED (top images), whereas the EV 4 lighting is tungsten (bottom)...although I've tried EV 4 with the LED with the same front-focused results.

snapsy wrote:
Was shooting with my week-old 5DM3+50mm f/1.4 today and noticed alot of OOF shots in low-light, even after having AF tuned my lens to -1 with lens align the day I got the body. Put the camera back on my lens align station and found that its focus is still perfect at -1 in EV 8 lighting (f/1.4 1/160 ISO 100) but it front-focuses in EV 4-5 lighting (f/1.4 1/13 ISO 100). I then tried it with my 50L and saw the same thing. Has anyone seen this?

Here are my 50 f/1.4 images. In these images the EV 8 lighting is 5000k LED (top images), whereas the EV 4 lighting is tungsten (bottom)...although I've tried EV 4 with the LED with the same front-focused results....Show more →

AF firmware does not have good AF sensor(s) reading...so it fails & jumps to its exception handling routine which might have some guessing or average preset logic to complete its algorithm. It is a normal design for any AF system. This explains why it won't fail with any speedlite focusing light on.

mttran wrote:
AF firmware does not have good AF sensor(s) reading...so it fails & jumps to its exception handling routine which might have some guessing or preset logic to complete its algorithm. It is normal design for any AF system. This explains why it won't fail with any speedlite focusing light on.

Well it's rather consistent though in the misses. I just tried the exact same test on my D800+50mm f/1.8 and it did the exact same thing as the 5DM3

snapsy wrote:
Tried it with my D3s with the same result as the 5DM3/D800, at least with tungsten. Did some online searching and apparently most PDAF systems do this. So odd that I never noticed it before.

The word on the street was that the 1Ds3 was the last dinosaur to have an AF system that did not take color temperature into account, that all newer systems did that. It's been my experience with the 5D3 that this is not the case, but I didn't conduct any scientific experiments. Your test now seems to support that.

Though not related to Canon, when I owned the Pentax K-5 it did this obviously in low light under Tungsten lighting. Perfect focus otherwise. The problem with the D800 is various speed lights add to the confusion when using AF assist on the speed light. It's a known issue with some Nikon bodies and the SB700/900 that the AF assist on the flash causes focus issues, but when disabled on the flash and AF assist is used on the body, the issue subsides.

It might be more related to color temperature than I originally thought...because even with my tungsten light source off the temperature drops 600K when I moved the LED light source further away to simulate low light...I'm guessing the lower temperature is from the reflections of the room affecting the tint. But it still appears the EV is having the biggest effect on the front focus, at least for the 5DM3.

To further isolate the low-light effects from the tungsten effects, here are two images with the same ~8EV, one with LED lighting (4750K) and the other with a tungsten CFL (3000K). Focus for both is spot-on at -1 AF tune, again indicating that the front-focus is from the lower EVs rather than tungsten.

stanj wrote:
The word on the street was that the 1Ds3 was the last dinosaur to have an AF system that did not take color temperature into account, that all newer systems did that. It's been my experience with the 5D3 that this is not the case, but I didn't conduct any scientific experiments. Your test now seems to support that.

I thought use of color in af in addition to contrast was only available in the 1dx. At least only the 1dx product description calls ou use of color.

snapsy wrote:
Tried it with my D3s with the same result as the 5DM3/D800, at least with tungsten. Did some online searching and apparently most PDAF systems do this. So odd that I never noticed it before.stanj wrote:
The word on the street was that the 1Ds3 was the last dinosaur to have an AF system that did not take color temperature into account, that all newer systems did that. It's been my experience with the 5D3 that this is not the case, but I didn't conduct any scientific experiments. Your test now seems to support that.jerrykur wrote:
I thought use of color in af in addition to contrast was only available in the 1dx. At least only the 1dx product description calls ou use of color.

That's not what I meant. It had been said a number of times that the AF mechanism takes color temperature, and its potential impact on AF accuracy / micro adjustment, into consideration in all cameras after the 1Ds3. That's not the same as saying that the AF uses color for Servo AF tracking.

I have never verified myself or through reliable sources that the claims are true, but when I mentioned on several occasions that the 1Ds3 front focuses in tungsten light I was told that I had a dinosaur, that all AF mechanisms since then (such as the 5D2) took the white point into consideration and adjusted AF accordingly. So I quietly shut up.

I've resolved the issue. In all my EV8 lensalign shots where AF tune was best at -1, I had an LED (4750K) or CFL tungsten (2750K) bulb at close proximity to the lensalign target, which creates a very high contrast target with high CA. For reasons which are not exactly clear to me yet, that high-contrast high-CA target produces a much different ideal AF tune value than all other AF scenarios I've since tried, including the same lensalign target but without the high-contrast lighting, plus all the real-world subjects I tried today, including some high-contrast targets in daylight sun. In all those other scenarios a +9 AF tune was best. And with that value, wow, the 5DM3 incredibly consistent with my 50mm wide open @ f/1.4, at all focus distances including mid-range distances (20 feet to 1/2 infinity), a feat no other PDAF camera I've ever had has achieved, including the 5D, 5DM2, D3, D3s, and D800.